Silica sols and method of making same



Patented June 24, 1952 UNIT ED ST OFFI SILICA SOLS AND METHOD'OF MAKING SAME" No Drawing. Application J anuary" 16', I950, SerialNo. 138,933

1'5'Glaims. (Cl; 252-313) This invention r lates; to. silic anic nitrogenvbaser compositions and; processes. or producing them, and; is more; particularly directed to table sols and watfir-disher le olid pr uct in both. of which silica. made'uprof dense, ultimate particles 15. to 130 millimicrons in diameter is intimately mixed with an. organic nitrogen base in which each organic substituent on the. nitrogen atom contains. no carbon atom farther removed from the nitrog n than consecutive carbon atoms and the total number or carbon atoms attached to the nitrogen exclusively through carbon is not more than 4- times the number of substituent groups, the. solsadditionally containing water, and is further directed to processes in which intimate admixtureof a. silica and a base of the above-described t es is effected in water, to make the sols anda sufficient portion of the water is then removed to make the solid products.

Silica in the form of. submicroscopic particles has many important. uses. In the treatment of textile fibers to impart. Slip resistance, as afiller in rubber, as an adjuvant to floor waxes for improving slip resistance, and as a-pigment in paints for special uses, for. instance, silica in, suclrsubmicroscopic form has advantageously'been used. customarily-the silica has beenaddedinthe form of an aqueous 501, because in the state 01' subdivision desired a dry silica product would be difficult to handle in large quantities due todusting. Furthermore, for the; above-mentioneduses and others, silica in such forms as glass, quartz, or sand cannot be disintegrated to the; desired fine particle size by mechanical grinding; except at prohibitive cost; hence chemical means have been employed, andone of, the most practicable forms of product to obtain=bychemical means is the aqueous sol.

The aqueous silica. sols h therto known have been expensive to-ship, handle, and store because of their large water content. Concentrations of SiOz much in excess of 20% by weight have been unstableagainstgelling; hence it, has-been necessary to handle about four parts. of water for each partof. silica. Recently, stable sols have been prepared containing 3.0% or. more of silica, by processes comprising addingincrementsof alkalistabilized silica sol to av heel of sol while evapcrating Water at elevated. temperature until 5 parts by weight of silica as increment. have been added per part of silica originally present asheel, as described in Bechtoldand Snyder application Ser. No. 65,536, filed December. 15, 1948, now Patent No. 2574302. When suchalkali-stabilized sols. are. concentrated much, beyond about. 40%

.then with an anion-exchanger.

2 S102, however they setirreversibly to gels. Also, the sols are detrimentally afiected; by freezing, so thatheatedstorage and Shipping facilities are required during winter weather.

Even a. small. amount-of alkali interferes with some uses for silica. sols, and. this disadvantage cannot. be. corrected. merely by neutralizing the alkali with. an acid, because: salts which act as electrolytes are. thus formed and electrolytes decrease. the stability of the sols. In my copending application, Ser. No. 9,7,09ofi1edJune 3, 1949,,now abandoned, of which the present application is a continuation-in-part, I have described processes for removing electrolytes from sols containing silica particles having an average diameter above 10 millimicrons, the.- processescomprising treating the sol, first. with. a, cation-exchanger and The cation-exchanger may be, incombination with an organic nitrogen. base such. as, a quaternary ammonium base or anorganic aminebaseand may thus replace the sodium ofa. sodium-stabilized sol by ion-exchange reaction. The organic nitrogen base may, alternatively, be added to the deionized sol. The: present application is directed to this aspect of my above-identified prior application.

It is" therefore an object; of the present invention to provide. processes for producing; novel aqueoussilica sols; Another object is to provide such solssinwhich the silica is. present in the form of dense ultimate particles from 15'. to millimicrons in diameter and there is no. substantial amount of alkali metal base orv electrolyte, present. Another object is, to provide such sols which are not detrimentally affected by exposure to. temperatures below the normal freezing point of water. Another object is to provide sols which are stabilized by. the presence thereinof an organic nitrogen base. Another object is to, provide novel solid, products which. are readily redispersible in water andwhich contain upwards of 50% by weight of silicain particulate form, intimately mixed. withan organic nitrogen base, and to provide processes for making such products. Further objects. will appear hereinafter.

The foregoing, and other objects are accomplished. according tQthiSinVention by making. an aqueous dispersionof silicainthe form, of dense, ultimate particles 15. to. l,3.0-.mil1imicrons in diameter, andintimately mixing with this dispersion an organic nitrogen base inwhich. each organic substituent. on the; nitrogen atomcontains no carbon.v atom farther removed-from thenitrogen than Scohsecutive carbon atoms and the total number. of. carbon atoms.- attachedto. the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups, whereby silica sols which are stable at high concentrations and over wide ranges of temperature are obtained, and are further accomplished by removtrated products, in contrast to, say, 20% SiOz sols, will be readily apparent. The solid products are,

from the practical viewpoint, soluble in water-that is, when they are added to water the silica is redispersed to form a stable sol with no more diificulty than is encountered in dissolving a truly soluble material in water.

The products of the invention enjoy the further advantage that they are free of alkali metal ions. If desired, the nitrogen base used in the products may be volatile, as is the case with methyl amine, and this may be boiled off at the point of use of the silica products, whereby a sol or solid silica product substantially free of electrolytes is the ultimate form of the silica. This freedom from alkali metal ions and ability to be free of electrolytes is highly advantageous in certain uses such as in the manufacture of phosphors for fluorescent lighting tubes and in catalyst manufacture.

The products of the invention have the further advantage that they are not detrimentally affected by wide changes of temperature. At low temperatures-below the normal freezing point of water, they either do not freeze, or if they do freeze, they redisperse to form stable sols upon thawing. On the other hand, they may be heated to 95 C. indefinitely Without precipitation or gelling.

THE SILICA DISPERSION TREATED sion of the silica in the form of dense, ultimate particles. It will be understood that the ultimate particles of silica need not necessarily be associated into chains or loose, open networks, but whether discrete or associated, the individual particles will be dense and will have an average diameter in the range from 15 to 130 millimicrons.

The silica dispersion used as the starting material in a process of this invention need not be a stable sol, since the treatment with organic nitrogen base will impart stability. It is sufiicient if the dispersion is a. suspension of particles of a character such that intimate mixing with the organic nitrogen base can be effected.

The ultimate particles of silica in silica dispersions to be treated must not be too small, and must be dense. The common silica sols made by neutralizing sodium silicate with an acid contain particles which are too small or not sufliciently dense to use without further treatment. Similarly, a silica sol prepared by ion-exchange, as in the Bird Patent 2,244,325, is composed of silica particles well below ten millimicrons in diameter. This S01 and other similar sols prepared by prior art methods and which have small diameter particles are quickly precipitated by a Change in pH to form a gel. They are accordingly unsuitable for use according to the present invention until further treated.

Now the silica sols of dense particles which it is preferred to use according to the present invention may be made by heating a silica sol, prepared by ion-exchange in the manner described in Bird 2,244,325 and stabilized with a small amount of alkali, to a temperature above 60 C. and adding further quantities of the same type of sol until at least five times as much silica has been added to the original quantity as was at first present. The product thus produced is stable against gelation at the pH of the present processes and it contains discrete silica particles having a molecular weight, as determined by lightscattering, of more than one-half million. The particle sizes can be made in excess of about fifteen millimicrons and-may range upwardly to,

say, about one hundred and thirty millimicrons.

The particles in a particular sol are surprisingly uniform in size, but the size can be varied depending upon the process conditions under which they were formed. The process as above outlined is fully set out in the application of Max F. Bechtold and Omar E. Snyder, in application Serial No. 65,536, filed December 15, 1948.

The particles of sol are quite dense and this may be shown by drying the particles and. then determining the amount of nitrogen adsorption. From the nitrogen adsorption it may be determined that the particles have a surface area not greatly in excess of that computed for the particle size as determined by electron micrograph. It will be evident that if the particles are not dense but rather are porous then the apparent surface as determined by nitrogen adsorption will be much higher than that expected from the particle diameters. Nitrogen adsorption, accordingly, affords an easy measure of the density of the particles. Summariz ing then, the preferred sols for use as starting materials according to the present invention have particles of such density that the surface area as determined by nitrogen adsorption is not greatly in excess of that computed for the particle size as determined by examination of an electron micrograph and the adsorption should not be more than about 30% greater than that computed from the apparent particle sizes. s

The method of determining the surface area by nitrogen adsorption is described in A New Method for Measuring the Surface Areas of Finely Divided Materials and for Determining the Size of Particles by P. H. Emmett in Symposium on New Methods for Particle Size Determination in the Subsieve Range in the Washington Spring Meeting of A. S. T. M., March 4, 1941.

According to the processes of Bechtold and Snyder it is preferred to produce sols which have a silica:alkali ratio of from 60:1 to :1. This refers to the Weight ratio of total silica expressed as $102 to total alkali expressed as NazO. The ratio may be even smaller and may go down to, say, 20:1 when the sols used are of moderately low concentration, say, below about 15% S102. It will be understood that if potassium is used as an equivalent of sodium it should be used at the same molar ratios. Such sols are then deionized by ion-exchange according to the processes of my prior application Ser. No. 97,090, now abandoned, to give the preferred aeonssa used for the preparation of asol containing dense particles.

Instead of the sols as: above described, which have extremely dense particles and very uniform particle size, one may use-instead the-somewhat less dense and non-uniform type of'product which can be made by precipitation of a silica gel and redispersion with alkali. Such a process is described, for instance, in the White Patent 2,375,738. The products produced by such processes contain considerableamounts of impurities. The deionizing processes of my above-mentioned prior application can accordingly be used to advantage in removing these impurities. The preferred processes ofthe invention and the preferred products of the invention, however, employ starting sols as previously described which are prepared by ionexchange because the rather great amountv of impurities and the lack of uniformity of the products just described leads to slightly less perfect results than can be obtained under the best conditions.

The products prepared byredispersion ofsilica gel ordinarily have a good deal higher nitrogen adsorption than would be indicated by apparent diameter. This shows considerable porosity. The nitrogen adsorption is about 50% greater than that computed.

Still other silica sols maybe used'and" it will be seen that it is important only that they have a particle size from about to 130 'millimicrons and they should be reasonably dense. It is this latter property which sharply distinguishes them from silica gels. Generally, it may be stated that for use in processes of the invention it is preferred to use sols, the particles ofwhich are uniform in diameter and have a diameter between fifteen millimicronsand one hundred and thirty millimicrons, the particles being further characterized by having a density suchthat; the size as indicated by nitrogenadsorption is not more than about 30% greater than thatcomputed from the apparent particle sizeas determined by the electron micrograph. More broadly, products may be used in which the nitrogen adsorption-indicates a size as much as, say, 100% greater than the size as determined by examination of an electron micrograph.

The silica dispersion to be treated with an organic nitrogen base according tothe; present invention may also be prepared by building up dense silica particles by processes described in Iler application Ser. No. 99,349, filed June 15, 1949, and then redispersing the precipitated products, or carrying the processes only to the points of precipitation. According to this method, an acid such as sulfuric is added to a hot (above 60 C.) solution of sodium. silicate over a period of time Thesodium ion concentration in the solution must not exceed one normal.

, An alternative method of efiecting such buildup of dense particles isdescribed in application Ser. No. 99,350, fileddune, 1; 5 1 949, by: G. B,

Alexander, R. K. Iler, and myself. According to this method dense silica. particles are made by heating asilica sol to. a. temperature above 60 C. and adding thereto asilicate solution. and enough of an acid to maintain a pH from eight to eleven, the heating to above 60 C. and the addition of silicate and acid being-continued at least. until the silica particles inthesol have reached an average particle size greater than 15 millimicrons.

The proportion of silica in the aqueous dispersion to be treated: with anorganic nitrogen base according to this invention may be varied considerably. It ispreferred to employ as high a concentration as feasible, and, as above indicated, difficulties due to precipitation are encountered when the concentration greatly exceeds 30% SiOz-by-weight, although after'adding the organic nitrogen basethe stability greatly improves; so. that a repeptization even of' precipitated silica particles is possible, provided the particles have not strongly. agglomerated after precipitation.

THE ORGANIC. NITROGEN BASE The organic nitrogen base employed in this invention may be ofv the aminetype or the quaternary ammoniurn type. In either case. it will contain organic groups attached to. the nitrogen atom. These organic groups maycontain one or more carbon atoms. and they maybe aliphatic, aromatic, or heterocyclic. A single organic group may have more than one point of. attachment to the nitrogen atom, and there may be one, two, or three organic groups on the nitrogen in the case of amines; quaternary ammonium bases will, of course, have four.

The structure of the organic nitrogen base must be such that each-organic substituent on thenitrogen atom contains-no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms. If the organic group is the normal amyl group, for example, the terminal carbon atom is 5 consecutive carbon atoms removed from the point of attachment. In the case of a phenyl group, the carbon of the benzene ring in the para position is only 4 carbon atoms removed from the point of attachment, because carbons may be counted; in either direction around the ring. Similarly; a benzyl group, containing a methyl substituent-on the benzene ring, comes-within the definition.

The total number ofcarbon atoms attached to the nitrogen atom of the base exclusively through carbon must not be more than 4 times the number of substituents. Thus, since quaternary bases contain 4 substituent carbon groups, the total number of carbons attached to the nitrogen exclusively through carbon is 16. Similarly, the largest number of' carbon atoms attached through carbon to the nitrogen of an amine will be 3 times 4, or 12,, and it can be as large as this only in the case of tertiary amines. In the case of a primary amine, with only one substituent organic group, the total number of carbon atoms possible is 4, It will be understood that there, may be morecarbon atoms than 4 times the number of substituent groups if the carbons are not consecutive. but are joined through, other atoms such as oxygen. Thus, airlines or quaternary ammonium bases substituted with polyethylene 0xide groups, in which the carbon groups are separated by oxygen atoms according to the arrangement N(CCO-) a: are operable even when the total number of carbons is above the limit set for direct o n cted carbons.-

I or heterocyclic groups.

Among the quaternary ammonium bases which may be used are those in which the s'ubstit'uents on the nitrogen are aliphatic, alicyclic, aromatic The groups may all be the same, or may be different. \Specific quaternary ammonium hydroxides which may be used are those in which the substituents are: Tetramethyl, tetraethyl, tetraisopropyl, tetra-n-propyl, tetraisobutyl, tetra-n-butyl, tetra-sec.-butyl, tetra-tert.-butyl, or combinations of the foregoing, methyl-triamyl, dimethyldiamyl, dimethyldiallyl, benzyltrim'ethyl, tetraethanol, trimethylethanol, phenyltrimethyl, trimethylcyclohexyl, n-methylpyridinium, n,n,-dimethyl piperidinium, n,n-dimethylmorpholinium.

Among the amine bases which maybe used are those in which the substituents on the nitrogen are aliphatic, aromatic, alicyclic, and heterocyclic groups. They may be primary, secondary. or tertiary. The groups may all be the same or may be different. Specific amines which may be used are those in which the substituents are: Mono-d1, or tri-methyl; mono-, di-, or tri-ethyl; mono-, di-, or tri-isopropyl; mono-, di-, or trin-propyl; mono-, di-, or tri-isobutyl; or combinations of the foregoing substituents; diethanolphenyl; mono-cyclopropyl; morpholine; mono-, di-, or tri-ethanolamine; pyrazine. Also useful are pyridine, piperidine, and similar cyclic compounds in which the nitrogen is in the ring. The aromatic amines are not preferred because of limited solubility.

THE PREPARATION 'OF THE SILICA-N BASE PRODUCTS To form a stabilized sol of the present invention, a suitable aqueous silica dispersion and a suitable organic nitrogen base, as above defined, are intimately mixed. The base is preferably water-soluble, and hence intimate mixture is obtained as it goes into solution. Compatibility of the more highly substituted amines with water is improved by the addition of a water-soluble organic solvent such as ethyl alcohol, acetone, or other water-soluble alcohols or ketones. Agitation is, of course, a preferred way to effect mixing.

The proportion of organic nitrogen base to silica may be considerably varied. Substantial effects on stability and similar properties of the silica are noticed when the nitrogen base is present in the amount of about 6% of the weight of silica. Larger amounts may advantageously be used, but it is preferred to limit the amount of nitrogen base to 50% of the weight of silica, since above this amount, depolymerization of the silica starts to occur to an undesirable extent.

To make the solid products from the sols, water is removed by any suitable method. Evaporation at reduced pressure, or elevated temperature, or both, may be used. Other methods, such as solvent extraction, may be employed in particular instances.

THE PRODUCTS The products of this invention, whether in sol or solid form, are characterized by containing silica in the form of dense, ultimate particles 15 to 130 millimicrons in diameter and by containing, in intimate association with the silica, an organic nitrogen base as above described. The size of the silica ultimate particles may be determined from electron micrographs and their density by comparing specific surface area measurements calculated from electron micrographs with specific surface areas calculated from nitrogen adsorption; preferably the latter should not exceed the former by more than 30%, or more broadly, by more than 100%.

The products of this invention are useful for a wide variety of purposes. As compared with similar silica products containing no organic nitrogen base they have improved stability against gelling and deterioration, particularly over a wide temperature range. They have improved compatibility with organic solvents and compounds, such as alcohols, acetone, and ketones generally, and like materials. They have substantially im proved film-forming properties.

Specific uses for which the products are particularly well adapted include treating textiles and textile fibers to improve slip-resistance and snag-resistance; as 'adjuvants to floor waxes for improving slip-resistance; as an ingredient of paints and coating compositions; as fillers in plastics and elastomers, as bodying-agents in greases; in adhesives; as carriers for insecticides and fungicide; as an intermediate in the preparation of phosphors for electrical purposes; in the preparation of catalysts; and in preparations generally where an easily-removable stabilizer is required.

Examples The invention will be more readily understood from the following illustrative examples.

Example 1 Tetramethyl ammonium hydroxide (10 parts) was added to 350 parts of a colloidal silica sol which had been prepared by a build-up process and possessed the following properties:

SiOz percent 28.2 Na do 0.02 S04 do 0.002 pH do 3.49 Molecular weight millions 16.2 Viscosity (25 C.) centistokes 4.55 Density g/ml. 1.19 Dn millimicrons 16.0 D5 dO Sc sq. meters/gram 43 SN do.. 181 Ds/DM 1.19 Sc/SN 0.79

where:

Molecular weight-:value determined by light scattering measurements.

Dn=Average particle diameter based on number and diameter of particles measured on an electron-micrograph of the sol.

Ds=Average particle diameter based upon uniform spheres having the same average surface as the range of particles measured on electron micrograph. c

S1v=Specific surface area as determined by nitrogen adsorption.

Sc=$pecific surface area as calculated from D5, the surface area average diameter. The value So is calculated from the formula Ds/Dn=A measure of the distribution of particle size.

Sc/SN=A measure of the porosity of the ultimate particles.

The stabilized sol resisted successive freezings at 12 C. and showed no indication of gelation upon three months storage at C. The sol was also concentrated until the S10: content had reached 50 per cent and the resulting sol was stable toward freezing and storage. Upon further concentration, the sol deposited a clear, tough film which cracked upon extensive drying. The dried product, however, dissolved readily in water to yield a clear stable silica sol.

Silica sols stabilized with this quaternary ammonium hydroxide were compatible with both acetone and alcohol and could be incorporated into sols of certain organic materials such as gelatin and polyvinyl alcohol.

Example? Morpholin'e (10 parts)- was added to the silica sol (350 parts) previously described in Example 1 to yield a s01 which was's't'able toward freezing at 12 CJand showed noindication of gelation upon storage at 95 C. This sol also poss'es'sed excellent compatibility with water-miscible organic compounds.

Examples Another example of-this invention would be to add ethanolamine (10 parts) to a sol (400 parts) containing dense silica particles which had'been prepared by a buildup process. A sol characterized by the following properties could be used:

SiOz per cent 30 Na do 0.02

S04 do 0.002 Molecular weight millions 54.2 Density degree B 24.9 pH 3.5 D11 millimicrons 28.4 D3 do 31.8 Sn ;sq. meters/gram 97 s ;---a-.-- d0 Ds Dn 1.12 sc/SN 1.00

The symbols are defined in Example 1.

The properties of this sol would be comparable to those of the sols described in Examples 1 and 2.

I claim:

1. A solid, water-dispersible composition comprising silica made up of dense, ultimate particles 15 to 130 millimicrons in diameter intimately mixed with an organic nitrogen base in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups.

2. A sol comprising an aqueous dispersion of silica made up of dense, ultimate particles 15 to 130 millimicrons in diameter intimately mixed with an organic nitrogen base in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups, the sol being substantially free of ions other than those of the organic nitrogen base.

3. A solid, water-dispersible composition comprising silica made up of dense, ultimate par ticles 15 to 13 0 millimicrons in diameter intimately mixed with an organic nitrogen base, selected from he group consisting of organic amines and quaternary ammonium hydroxides, in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups,

4. A sol comprising an aqueous dispersion of silica made up of dense, ultimate particles 15 to millimicrons in diameter intimately mixed with an organic nitrogen base, selected from the group consisting of organic amines and quaternary ammonium hydroxides, in which each organic substituent on the nitrogen atom contains nocarbon atom farther removed from the nitrogen than 5 consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups, the sol being substantially free of ions other than those of the organic nitrogen base.

5. A solid, water-'dispersiblecomposition comprising silica made up of dense, ultimate particles 15 to 130 millimicrons in diameter intimately mixed with an organic nitrogen base, selected from the group consisting of organic amines and quaternary ammonium hydroxides, in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups, the proportion of nitrogen base being from 6 to- 50% of the weight of the silica.

6. A sol comprising an aqueous dispersion of silica made up of dense, ultimate particles 15 to 130 millimicrons in diameter intimately mixed with an organic nitrogen base, selected from the group consisting of organic amines and quaternary ammonium hydroxides, in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups, the proportion of nitrogen base being from 6 to 50% of the weight of the silica, and the sol being substantially free of ions other than those of theorganic nitro en base.

'7. In a process for making silica sols the steps comprising making an aqueous dispersion of silica in the form of dense, ultimate particles 15 to 130 millimicrons in diameter and intimately mixing with this dispersion an organic nitrogen base in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups.

8. In a process for making silica sols the steps comprising making an aqueous dispersion of silica in the form of dense, ultimate particles 15 to 130 millimicrons in diameter and intimately mixing with this dispersion an organic nitrogen base in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups, the proportion of nitrogen base being from 6 to 50% of the weight of the silica, and the sol being substantially free of ions other than those of the organic nitrogen base.

9. In a process for making solid, water-dispersible compositions the steps comprising making an aqueous dispersion of silica in the form of dense, ultimate particles 15 to 130 millimicrons in diameter and intimately mixing with this dispersion an organic nitrogen base in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups, and removing water from the mixture until it solidifies. r

10. In a process for making solid, water-dispersible compositions the steps comprising making an aqueous dispersion of silica in the form of dense, ultimate particles 15 to 130 millimicrons in diameter and intimately mixing with this dispersion an organic nitrogen base in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms and the total number of carbon atoms attached to the nitrogen exclusively through carbon is not more than 4 times the number of substituent groups, the proportion of nitrogen base being from 6 to 50% of the weight of the silica, and removing water from the mixture until it solidifies.

11. A sol comprising an aqueous dispersion of silica made up of dense, ultimate particles 15 to 130 millimicrons in diameter intimately mixed with an organic quaternary ammonium base in which each organic substituent on the nitrogen atom contains no carbon atom farther removed from the nitrogen than 5 consecutive carbon atoms' and the total number of carbon atoms at tached to the nitrogen exclusively through care bon is not more than 4 times the number of sub= stituent groups.

12. A sol comprising an aqueous dispersion of silica made" up of dense, ultimate particles 15 to millimicrons in diameter intimately mixed with tetramethyl ammonium hydroxide, the sol being substantially free of ions other than those of the tetramethyl ammonium hydroxide.

13. A s01 comprising an'aqueous dispersion'of silica made up of dense, ultimate particles "15 to 130 millimicrons in diameter intimately mixed with ethylamine, the sol being substantially free of ions other than those of the ethylamine.

14. A sol comprising an aqueous dispersion. of silica made up of dense,-ultimate particles 15 to 130 millimicrons in diameter intimately mixed with triethanolamine, the sol being substantially free of ions other than those of the triethanolamine. v Y

15. A sol comprising an aqueous dispersion of silica made up of dense, ultimate particles 15 to 130 millimicrons in diameter intimately mixed with morpholine, the sol being substantially free of ions other than those of the morpholine.

' FREDERICK J. WOLTER.

.. REFERENCES CITED The following references are of record in the file of this patent:

UNITED sTATEs PATENTS Pierce May 2, 1950 

1. A SOLID, WATER-DISPERSIBLE COMPOSITION COMPRISING SILICA MADE UP OF DENSE, ULTIMATE PARTICLES 15 TO 130 MILLIMICRONS IN DIAMETER INTIMATELY MIXED WITH AN ORANIC NITROGEN BASE IN WHICH EACH ORGANIC SUBSTITUENT ON THE NITROGEN ATOM CONTAINS NO CARBON ATOM FARTHER REMOVED FROM THE NITROGEN THAN 5 CONSECUTIVE CARBON ATOMS AND THE TOTAL NUMBER OF CARBON ATOMS ATTACHED TO THE NITROGEN EXCLUSIVELY THROUGH CARBON IS NOT MORE THAN 4 TIMES THE NUMBER OF SUBSTITUENT GROUPS. 